The standard for WLAN (wireless local area network) technology is being developed as IEEE (institute of electrical and electronics engineers) 802.11 standard. IEEE 802.11a/b uses an unlicensed band on 2.4 or 5 GHz. The IEEE 802.11b provides a data rate of 11 Mbps, while the IEEE 802.11a provides a data rate of 54 Mbps. IEEE 802.11g applies OFDM (orthogonal frequency division multiplexing) on 2.4 GHz to provide a data rate of 54 Mbps. IEEE 802.11n applies MIMO-OFDM (multiple input multiple output-OFDM) to provide a data rate of 300 Mbps for 4 spatial streams. The IEEE 802.11n supports channel bandwidths up to 40 MHz. In this case, a data rate of 600 Mbps is provided.
Currently, IEEE 802.11af standard is being developed to regulate an operation of an unlicensed device on TVWS (TV whitespace) band.
A TV white space (TV WS) is a frequency assigned to a broadcast TV, includes a UHF (ultra high frequency) band and a VHF (very high frequency) band, and means the frequency bands allowed to be used for an unlicensed device under the condition that communications of a licensed device operating on the corresponding frequency bands are not interrupted. In particular, a device operating as an unlicensed device can use an available channel or frequency band currently not used by a licensed device on the corresponding frequency band. In this case, the licensed device may include one of a TV, a wireless microphone and the like. And, the licensed device may be called a licensed user, an incumbent user, a primary user or the like.
An unlicensed device desiring to use the TV WS band should provide a protective function for a licensed device. Hence, before starting a transmission of a signal on the TV WS band, it is mandatory for the unlicensed device to check whether a licensed device occupies the corresponding band in order to obtain available channel information.
To this end, the unlicensed device may check whether the corresponding band is used by a licensed device by performing a spectrum sensing. Spectrum sensing mechanism may include one of an energy detection scheme (i.e., a scheme of determining that an incumbent user is using a corresponding band if a strength of a received signal is equal to or greater than a predetermined value), a feature detection scheme (i.e., a scheme of determining that an incumbent user is using a corresponding band if a digital TV preamble is detected), and the like. If a strength of a signal received on a specific channel is equal to or greater than a predetermined value or a DTV (digital TV) preamble is detected, the unlicensed device can determine that the licensed device is currently using the specific channel. If it is determined that a licensed device is using a channel right adjacent to a currently used channel, a transmission power of an unlicensed device may be lowered depending on a level of an interference radiating into an adjacent band.
Yet, in case that an unlicensed device depends on a frequency sensing only to obtain available channel information from TV WS band, a burden due to a sensing for an operation on the TV WS band is further put on the unlicensed device and a corresponding procedure may be delayed. Hence, the unlicensed device accesses a geo-location database through Internet or a dedicated network and is then able to obtain channel list information on a list of channels available in a corresponding area. In this case, the geo-location database is configured to store and manage information on registered licensed devices and channel use information on a channel dynamically changing depending on geo-locations and channel used times of the licensed devices.